| The edge of Tarim basin of south Xinjinag is one of the most important cotton production regions in China. Unreasonable N fertilization application appear in high-yield-cotton system in this region.It causes the deereasing of nitrogen use efficiency. The effect of different nitrogen management strategies on cotton growth and how to make cotton nitrogen diagnosis by topdressing recommendation with SPAD-502 were tested by field and pot experiments at Shaya and Akesu counties of southern Xinjiang in 2007 and 2008. A system for cotton topdressing recommendation based on SPAD was established and the model was tested. Effects of nitrogen rates on photosynthetic characteristic, accumulation of dry matter, absorption of N,P,K, cotton yield and quality were studied simultaneously. The main results are as follows:1. The results showed that leaf N content and SPAD value varied greatly at different leaf positions, to increase N application can increase the leaf N content and SPAD value, which at the mean time decreased the difference between the different position leaves. The highest sensitivity of SPAD value to N rates was the fourth leaf from the top to bottom, and the second leaf was the lowest, while the first and the third leaves varied within the varieties. The SPAD value of the fourth leaf from the top was most correlated with whole leaf and plant N concentration among the top four leaves through the bud stage, initial-flowering stage and full-flowering stage. The CV of SPAD value of the fourth leaf from the top was smallest under optimal N level. It was suggested that the fourth leaf from the top was the most ideal indicator when using SPAD value or leaf color difference to diagnose N status during cotton growing periods.2. The SPAD values increased significantly with N rate increasing, and it could be used as indices for diagnosis of the plant nitrogen nutrition. The SPAD values of the 4th fully-expanded leaf from the top on the main stem were more sensitive to the N rate increasing, while the 1st fully-expanded leaf less sensitive. The relative differences of SPAD values between L4 and L1 were related to plant leaves'nitrogen concentrations, which could indicate plant leaves nitrogen status of cotton under varied nitrogen supply levels during different growth stages.3. The SPAD value of the 4th expanded leaf at the stages of flowering, full flowering, full ball periods were significantly correlated with N rate applied and cotton yield. The critical values of SPAD were 61.8,61.3 and 62.9 separately as nitrogen diagnostic index during the three main growth stages, which meant the N rate was not enough if the SPAD was under the above value. Based on the above relationship, a cotton nitrogen fertilizer recommendation model was established, which could be used to recommend the rate of nitrogen topdressing.4. The optimized N rate based on SPAD diagnosis model could be reduced about 9.7%-17.1% and 22.5%-22.7% compared with normal N fertilizer application on sandy loam and clay loam soils respectively, and at the same time, there was no N deficiency symptom during growing period of cotton.5. Proper nitrogen rate could improve photosynthetic characteristics of cotton and the chlorophyll content, delayed senescence of leaves, raised net photosynthesis rate; cotton leaves area and LAI were enhanced with reasonable N applied, and LAI decreased rapidly in later stage with excessive N rate. Shedding rate of cotton stem leaf declined when N rate was 258.75kg·ha-1. Cotton yield increased when N rate applied was among 0-283.8kg·ha-1, and decreased until N rate applied exceed 283.8kg·ha-1. 6. The influence of nitrogen rate applied on fiber length, uniformity, strength and Micronaire value were different. Fiber length and uniformity increased with nitrogen application at the level 0-283.8kg·ha-1 and decreased until N rate applied exceed 283.8kg·ha-1.The trend showed that fiber strength and the Micronaire value decreased with the increase of nitrogen application.7. The dynamic accumulation model of dry cotton matter could be described by Logistic equation. The accumulation of dry matter was sensitive to nitrogen level. The starting time and ending time of the fast growth stage of biomass accumulation of cotton varied with different fertilizer rate. The varying parameters of average speed, the highest speed of biomass accumulation in fast growing periods affected the progress of reproduction growth and nutrient growth. Earlier stage of speediness growth, shorter period and quicker increase rate were the characteristics at 315kg·ha-1 and 210kg·ha-1 nitrogen level on sandy loam and clay loam soils respectively. It showed that nitrogen rate could accelerate the accumulation of biomass in earlier growing stage. The nitrogen rate and the application time were very important for the highest cotton yield.8. The accumulations of N, P2O5, K2O in cotton plant could be described by Logistic curve equation. The treatments with the highest accumulation of N , P2O5 , K2O were 315 kg·ha-1 and 210kg·ha-1 nitrogen level for sandy loam and clay loam soils respectively, and the cotton yields were the highest and the parameters of dynamic accumulation model were more harmonious than that of other treatments. The starting time of speediness growth stage were from 20-24 day for N, 24-28 days for P2O5, 20-22 days for K2O after bud emergence on sandy loam soil while from 21-24 day for N, 25-27 days for P2O5 , 21-23 days for K2O on clay loam soil. The time of nutrient uptake fastest was at about 36-41 days for N , 38-42 days for P2O5 , 40-45 days for K2O on sandy loam and at about 39-44 days for N , 39-46 days for P2O5 , 40-44 days for K2O on clay loam soil after bud emergence. The order of the greatest nutrient accumulation speed was N , P2O5 and K2O.
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